The Mariner 4 mission, the second of two Mars flyby attempts in 1964 by NASA, was one of the great early successes of the Agency, returning the very first photos of another planet from deep space.

Using a new all-metal shroud, the spacecraft lifted off without any problems and was successfully boosted toward Mars by the Agena D upper stage. A single midcourse correction on 5 December 1964 ensured that the spacecraft would fly between 8,000 and 9,660 kilometers from the Martian surface. Approximately 40 minutes prior to closest approach (which was at 01:00:57 UT on 15 July 1965 at a range of 9,846 kilometers), the TV camera began taking the first of 21 images (plus 22 lines of a twenty second) through red and green filters.

About 1.25 hours after the encounter, Mariner 4 dipped behind the right-hand side of Mars (as viewed from Earth) in order to refract its radio signals through the Martian atmosphere. Data indicated that surface pressure was quite low -- future Mars landers would have to be equipped with retro-rocket engines in addition to parachutes in order to safely land on the Martian surface.

The probe detected daytime surface temperatures of about -100°C. A very weak radiation belt, about 0.1 percent that of the Earth's, was also detected by Mariner 4. The day after the closest encounter, Mariner 4 began transmitting its photos back to Earth.

The images clearly showed Mars to be an ancient Moon-like body with widespread cratering. Given the thin atmosphere, scientists believed it unlikely that Mars harbored any life.

NASA maintained contact with the spacecraft until 1 October 1965, when the probe was 309 million kilometers from Earth. Two years later, in October 1967, the spacecraft was reactivated for attitude control tests in support of the Mariner 5 mission to Venus, which used a similar spacecraft bus. Final contact was lost on 21 December 1967.